There is an increase in the use of VoIP (Voice over Internet Protocol), VoPN (Voice over Packet Network), applications such as streaming, and the like for transmitting and receiving audio and visual data between devices via a communication network such as an IP (Internet Protocol) network. As an example of such an application, IP telephones that are capable of transmitting and receiving packets via an IP network are rapidly becoming widespread.
An IP telephone converts, for example, an analog audio signal based on the speech of a user into a digital signal, packetizes the audio signal converted into the digital signal, and transmits and receives the packetized audio signal to and from an IP telephone of a communication partner via an IP network. Therefore, in order to initiate communication between IP telephones, the IP address of the IP telephone of a communication partner (called party) must be identified when the IP telephone of the calling party initiates a call. Recently, a standard configuration of an IP telephone system involves the use of an SIP (Session Initiation Protocol) server in order to identify the IP address of a called IP telephone.
With a system using an SIP server, upon activation, each IP telephone registers, in the SIP server, an IP address assigned to itself. When initiating a call, an IP telephone then asks the SIP server about the IP address of an IP telephone of the communication partner (called party). In response to the inquiry from the calling IP telephone, the SIP server identifies the IP address of the called IP telephone, and using the identified IP address, notifies the called IP telephone to expect an incoming call. When there is a response to the notification from the called IP telephone, the SIP server establishes a call connection between the calling IP telephone and the called IP telephone. Consequently, IP communication (call) is initiated between the calling IP telephone and the called IP telephone.
Meanwhile, for the purpose of using IP telephones without being restricted by the point of use, there is a demand for the realization of an IP telephone service over a mobile communication network such as a FOMA (registered trademark) network which can be used over a wide area. However, with existing mobile communication networks (so-called 3G networks), always-on IP connections of IP terminals (IP telephones) are not allowed due to restrictions in address resources. Therefore, each IP telephone is required to temporarily establish an IP connection as necessary, and utilize applications such as a Web browser or e-mail using a dynamically assigned IP address.
In such circumstances, since an IP address cannot be constantly assigned to all IP telephones, the IP addresses of all IP telephones cannot be registered in an SIP server. When an IP telephone whose IP address is not registered receives an incoming call, the SIP server returns an error to the calling IP telephone. Consequently, there are situations where communication with an IP telephone whose IP address is not registered in the SIP server cannot be performed.
In consideration thereof, an SIP server is proposed which, when an IP telephone with an unregistered IP address receives an incoming call, notifies the called IP telephone to expect an incoming call using an SMS (Short Message Service) on a switched telephone network instead of returning an error to the calling IP telephone (refer to Patent Documents 1 and 2). When an SMS notification is received, the called IP telephone establishes an IP connection to a mobile communication network to acquire an IP address, and registers the acquired IP address to the SIP server that made the notification regarding the incoming call.
Upon confirming the registration of an IP address of the called IP telephone, the SIP server uses the registered IP address to once again notify the called IP telephone to expect an incoming call. When there is a response to the notification from the called IP telephone, the SIP server establishes a call connection between the calling IP telephone and the called IP telephone. Such a technique enables IP communication (call) between IP telephones via a mobile communication network regardless of whether or not an IP address is registered in the SIP server. Examples of such a technique include, for example, Japanese Patent Laid-Open No. 2007-20000 and Japanese Patent Laid-Open No. 2006-180372.